1. Field of the Invention
The present invention relates to a receiver coil loop, in particular to a receiver coil loop for a magnetic resonance imaging system.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) is an imaging modality, wherein the signals generated by nuclear magnetic resonance are reconstructed for imaging. More specifically, the basic principle of magnetic resonance is that: for atomic nuclei with odd numbers of protons, e.g. the hydrogen nucleus that permeates a human body, the positively charged protons thereof generates spinning movements to generate a magnetic moment, serving as a small magnet. The arrangement of the spin axes of these small magnets is normally irregular (random). But in a homogenous strong magnetic field the spin axes of the small magnet will be rearranged depending on the direction of the magnetic field lines. In this state, excited by a radio frequency (RF) pulse at a specific frequency, the hydrogen nucleus, serving as a small magnet, absorbs a certain amount of energy so as to resonate, causing the magnetic resonance phenomenon to occur. Once the transmitting of RF pulses is stopped, the excited hydrogen nucleus will release the absorbed energy gradually, and then the phase and energy level will resume to the state before the excitation. Different from the imaging principle in X ray, CT, etc., MRI causes no radiation harm to human body, and therefore it has provided a broad research field for clinical applications.
A magnetic resonance imaging system basically includes a basic field magnet, a gradient field coil system, an RF coil system, a control unit for executing a sequence, and an image processing and display system. The gradient coil system is employed to modify the main magnetic field, and to generate a gradient magnetic field. The gradient magnetic field provides the possibility for three-dimensional coding of the magnetic resonance signals in the human body for spatial orientation, though the magnetic field strength thereof is only several hundredths of the main magnetic field. The RF coil system includes a transmitter coil and a receiver coil. The transmitter coil transmits pulses into the human with proper RF energy for excitation, serving as a short wave transmitting channel and a transmitting antenna, so the hydrogen nucleus (atomic nucleus with an odd number of protons) within the human body receive the pulse, serving as a radio receiver. After the transmission of the pulses (excitation) is stopped, the hydrogen nucleus within human body serves as a shortwave transmitter, whereas the MR signal receiver serves as a radio receiver to receive magnetic resonance signals. The functions of the magnetic resonance signal receiver are realized by the receiver coils.
The aforementioned receiver coil converts the magnetic signals transmitted from the human body into electrical signals, which are then transferred to the image processing and display system via a cable for further image reconstruction processing. To maximize the signals, the frequency of the received signals should equal to the frequency of the signals transmitted by the atomic nucleus, i.e. resonance should be achieved. For hydrogen atoms (protons) the resonant frequency is 14.6 MHz in a magnetic field of 0.35 T. Since the magnetic field strength of the manufactured basic field magnet has a certain deviation, centered around 0.35 T the resonant frequency of protons also shifts slightly around the resonant frequency.
Thus, in the MRI system, if the receiver coil is to operate properly, the receiver coil must work properly within a frequency band with the resonant frequency (resonance frequency ±100 kHz) at the center. Generally, this object is achieved by discrete circuitry, such as the circuitry described in the document Siemens Internal Document Part Number: 7100303, 7100394. The so-called discrete circuitry indicates that the fixed capacitors respectively employed by the tuning loop and the detuning loop are different. In FIG. 1, the fixed capacitors employed by the tuning loop are Cp and Cdetune, whereas the counterpart employed by the detuning loop is Cdetune. The opposite of discrete circuitry is combined circuitry. Combined circuitry indicates that the fixed capacitors employed by the tuning loop and the detuning loop are the same. The theory of the discrete circuitry of the receiver coil can be as illustrated in FIG. 1. As in FIG. 1, three loops are included, which are a tuning loop, a detuning loop and a matching loop, respectively.
In the discrete circuitry in FIG. 1, the detuning loop has an inductor Ls, a diode D, and a detuning capacitor Cdetune. The inductor Ls is connected in series to the diode D, and is connected in parallel with the detuning capacitor Cdetune. The detuning loop is mainly to maintain the receiver coil in a non-working state when the transmitter coil is transmitting signals, i.e., so that the diode D is ON, and the inductor Ls and the detuning capacitor Cdetune are in the resonant state. According to the resonance theory, now the detuning loop disconnects from the external loop; thus, the receiver coil does not work. When the tuning loop works, the diode D is non-conducting, thus the tuning loop is composed of a tuning capacitor Ctuning, the detuning capacitor Cdetune, a load resistance R and the inductors Ls connected in series, and a capacitor Cp further connected in parallel with these components, so as to cause the loop to resonate at the central frequency of the receiver coil, by adjusting the tuning capacitor Ctuning. The tuning capacitor is formed by a fixed capacitor C and an electrolytic capacitor Ctuning′ connected in parallel, as shown in FIG. 2. The matching lines may include the tuning capacitor Cs and the tuning loop connected in series, so that the signal transmission is maximized as the signals are prevented from reflecting within the tuning loop, when the frequency of the tuning loop matches the central frequency of the receiver coil. These three loops cooperate to fulfill the requirements for the receiver coil to produce resonance within an acceptable resonant frequency range.
In the resonant state, the detuning capacitor Cdetune and the tuning capacitor Ctuning of the discrete circuitry hold respective portions of the inductive reactance. In other words, the inductor L, the detuning capacitor Cdetune, the tuning capacitor Ctuning, and the capacitor Cp are formed into an LC loop. The LC loop satisfies ω√{square root over (LC)}=1, wherein ω is the angular velocity at the center frequency, L is inductance. Both are constant values; therefore the total capacitive reactance C of the tuning circuit is also a constant value. Since the total capacitive reactance C is formed by Cdetune, Ctuning, and Cp connected in series, and the capacitive reactance has a tendency of being decreased as additional items enter into the serial connection, the capacitances of the tuning capacitor Ctuning, the detuning capacitor Cdetune, and the capacitor Cp are all greater than the total capacitive reactance C. In addition, since the detuning capacitor Cdetune holds a portion of the inductive reactance, the inductive reactance assigned to the tuning capacitor Ctuning is reduced, resulting in an increased capacitance value for the tuning capacitor Ctuning. In the tuning loop, a value of the tuning capacitance Ctuning that is too large may lead to a smaller tuning range, and since the field strength of the basic field magnet cannot be precisely 0.35 T (due to manufacturing tolerances), the resonant frequency may be slightly altered. Each receiver coil loop must be adjusted individually, i.e., it is necessary to choose and adjust a tuning capacitor Ctuning so that the receiver coil works properly within a frequency band with the resonant frequency at the center, which leads to a rather low production efficiency for the current receiver coil.
The prior art has attempted to employ the method shown in FIG. 3, to reduce the inductive reactance held by the detuning capacitor Cdetune. In the detuning state, when the diode is ON, the capacitor C′ is disabled, thus the detuning circuit is the same as that shown in FIG. 1. However, in the tuning state, Ls is in effect connected in series to C′, and then further connected in parallel with Cdetune. The parallel connection between the capacitors C′ and Cdetune increases the capacitance of the detuning loop, and reduces the inductive reactance thereof. Since the inductive reactance held by the tuning capacitor Ctuning is increased, the capacitance value thereof may be reduced, and thereby the tuning range is broadened to a certain extent. However, it still cannot meet the requirements for a large-scale tuning, and the loss of the circuitry is increased, since Ls is still working during tuning.